Many types of machines, particularly material handling machines, grapple tractors and the like used in the construction industry, comprise some form of operable apparatus and a cab in which the operator sits which are mounted on a base, such as a vehicle body. The operable apparatus may include, for example, a boom to which a grapple, magnet or clam shell or the like is attached. The operator may use controls located in the cab to direct the movement of the apparatus, for example to pick up and transfer material from one place to another.
In some machines the position of the operator cab is fixed to the machine body, which can be disadvantageous in some working environments as the operator's view of the working environment can be impeded by the position of the boom and/or apparatus. To overcome this problem fixed and hydraulic cab risers have been designed to allow the operator to raise the cab to a desired height above the body of the machine to give a better view of the working area.
One example of hydraulic riser apparatus is described in EP-A-0826578 which uses a number of inverted hydraulic cylinders and support arms arranged in a parallelogram linkage between a brace and a bracket member secured to the machine body to elevate the cab. The stability of the apparatus is enhanced in that the hydraulic cylinder is in a retracted position, when the cab is in a raised position which limits unwanted movement or wobble during operation of the machine. Additional hydraulic cylinders are used to increase the amount of lifting force.
However, this type of arrangement only provides means for lifting the cab vertically. JP-A-2007001469, on the other hand, describes a system which enables the cab to not only be raised, but also be lowered to the ground to facilitate operator entry and exit. A parallel linkage mechanism is attached to an arm at the back of the cab, which linkage comprises a pair of arms and hydraulic cylinder. This enables the cab to be moved forward and down as well as up. The arm can be replaced by a second parallel linkage, with a coupling element to the linkages.
However, as the system is designed to extend the cab forward and down to ground level, the structure is a complex design which adds to the cost and difficulty of manufacture. Furthermore the system requires attachment to a revolving super structure.
It is not always required to enable the cab to be extended down to the ground as in the aforemention prior art system, although it is desirous to be able to move the cab forward in a horizontal direction and in a vertical upward direction.